The invention relates to electrophotography. More particularly, it relates to novel polymers and a novel photoconductive element that contains a polymeric electrical charge barrier layer.
Photoconductive elements useful, for example, in electrophotographic copiers and printers are composed of a conducting support having a photoconductive layer that is insulating in the dark but becomes conductive upon exposure to actinic radiation. To form images, the surface of the element is electrostatically and uniformly charged in the dark and then exposed to a pattern of actinic radiation. In areas where the photoconductive layer is irradiated, mobile charge carriers are generated which migrate to the surface and dissipate the surface charge. This leaves in nonirradiated areas a charge pattern known as a latent electrostatic image. The latent image can be developed, either on the surface on which it is formed or on another surface to which it is transferred, by application of a liquid or dry developer containing finely divided charged toner particles.
Photoconductive elements can comprise single or multiple active layers. Those with multiple active layers (also called multi-active elements) have at least one charge-generation layer and at least one ntype or p-type charge-transport layer. Under actinic radiation, the charge-generation layer generates mobile charge carriers and the charge-transport layer facilitates migration of the charge carriers to the surface of the element, where they dissipate the uniform electrostatic charge and form the latent electrostatic image.
Also useful in photoconductive elements are charge barrier layers, which are formed between the conductive layer and the charge generation layer to restrict undesired injection of charge carriers from the conductive layer. Various polymers are known for use in barrier layers of photoconductive elements. For example, the patent to Hung, U.S. Pat. No. 5,128,226, discloses a photoconductor element having an n-type charge transport layer and a barrier layer, the latter comprising a particular vinyl copolymer. Steklenski, et al., U.S. Pat. No. 4,082,551, refers to Trevoy, U.S. Pat. No. 3,428,451, as disclosing a two-layer system that includes cellulose nitrate as an electrical barrier. Bugner et al. , U.S. Pat. No. 5,681,677, discloses photoconductive elements having a barrier layer comprising certain polyester ionomers. Pavlisko et al, U.S. Pat. No. 4,971,873, discloses solvent-soluble polyimides as polymeric binders for photoconductor element layers, including charge transport layers and barrier layers.
The known barrier layer materials have certain drawbacks, especially when used with negatively charged elements having p-type charge transport layers. Thus, a negative surface charge on the photoconductive element requires the barrier material to provide a high-energy barrier to the injection of positive charges (also known as holes) and to transport electrons under an applied electric field. Many known barrier layer materials are not sufficiently resistant to the injection of positive charges from the conductive support of the photoconductive element. Also, for many known barrier materials the mechanism of charge transport is ionic. The ambient humidity affects the water content of the barrier material and, hence, its ionic charge transport mechanism. A need exists for charge barrier materials that transport charge by electronic rather than ionic mechanisms and that, therefore, are not substantially affected by humidity changes.
Still further, a number of known barrier layer materials function satisfactorily only when coated in thin layers. As a consequence, any irregularities in the coating surface, such as bumps or skips, can alter the electric field across the surface. This in turn can cause irregularities in the quality of images produced with the photoconductive element.
Accordingly, a need exists for a negatively chargeable photoconductive element having a p-type photoconductor, and including an electrical barrier layer that can be coated from an aqueous or an organic medium, that has good resistance to the injection of positive charges, that can be sufficiently thick that minor surface irregularities do not substantially alter the field strength and that resists hole transport over a wide humidity range. Still further a need exists for photoconductive elements of which the barrier layer is substantially impervious to or insoluble in solvents used for coating other layers, e.g., charge generation layers, over the barrier layer. In accordance with the present invention, a novel photoconductive element and certain novel polyamides that meet such needs are provided.
Photoconductive elements comprising a photoconductive layer formed on a conductive support such as a film, belt or drum, with or without other layers such as a barrier layer, are also referred to herein, for brevity, as photoconductors.
The photoconductive element of the invention comprises an electrically conductive support an electrical barrier layer and, solvent-coated over the barrier layer, a charge generation layer that is capable of generating positive charge carriers when exposed to actinic radiation. The electrical barrier layer, which restrains the injection of positive charge carriers from the conductive support, comprises a condensation polymer having as a repeating unit a planar, electron-deficient, aromatic tetracarbonylbisimide group that transports charge primarily by electronic rather than ionic transport mechanisms. This barrier layer polymer is substantial insoluble in the solvent for the charge generation layer under the coating conditions employed. More specifically, in the photoconductive element of the invention, said barrier layer comprises a condensation polymer having covalently bonded as repeating units in the polymer chain aromatic tetracarbonylbisimide groups of the formula: 
wherein Ar1 and Ar2 represent, respectively, tetravalent and trivalent aromatic groups of 6 to 20 carbon atoms.
Preferably, the barrier layer polymer in the photoconductive element of the invention is a polyester-co-imide, polyesterionomer-co-imide, or polyamide-co-imide that contains an aromatic tetracarbonylbisimide group, and has the formula: 
wherein Q represents one or more groups selected from
(a) alkylenedioxy, aromatic dicarboxy and aromatic diamino groups having 2 to 36 carbon atoms; 
and wherein Ar, Ar1 and Ar2 independently represent tetravalent aromatic groups having 6 to 20 carbon atoms; Ar3 represents a trivalent aromatic group having 6 to 12 carbon atoms; R1 and R2 independently represent alkylene or alkyleneoxy groups having 2 to 12 carbon atoms; L1, L2, L3, L4, L5 and L6 independently represent O, CO, CO2, or NH; and Z1 and Z2 independently represent an alkylenedioxy or alkylenediamino group having 2 to 36 carbon atoms; X is O, C(CF3)2, Sxe2x95x90O or SO2; and x and y represent mole fractions, x being the mole fraction of the group that contains Ar1 and y being the mole fraction of the group, Z1; and wherein x is 0.05 to 1 and y is 0 to 0.95.
The present invention also includes as novel compositions of matter the novel polyamides defined above.